Manufacture of acetic anhydride



Patented Oct. 13, 1942 MANUFACTURE OF ACETIC ANHYDRIDE Henry Dreyfus, London, England, assignor to Celanese Corporation Delaware of America, a corporation of No Drawing. Application December 29, 1938, Serial No. 248,206. In Great Britain February 6 Claims. (01. 260-546) run off from th reaction zone continuously or This invention relates to the manufacture of acetic anhydride.

The oxidation of acetaldehyde is a process which has been widely employed in the manufac ture of acetic acid, although it is known that acetic anhydride is also produced in the process, probably as an intermediate product. However, while in the manufacture of acetic acid, recovery in a pure form of the acetic acid offers no dimculty, when the manufacture of acetic anhydride is attempted a serious difficulty is met with, namely the tendency of the anhydride formed to be hydrolysed by the water simultaneously pro-. duced. Efficient methods of separating vaporous mixtures of acetic anhydrideand water, such as are produced by the thermal dehydration of acetic acid, are very well known, but, while at first sight it would seem natural to apply these methods to the separation of mixtures of acetic anhydride and water produced by oxidising acetaldehyde, it is found in practice that the,

problem is not adequately solved by these me'th- -ods. This is because the separation processes involve evaporation of the liquid mixture containing acetic anhydride and water and during this evaporation step a considerable quantity of the acetic anhydride is hydrolysed. Yields of anhydride in the oxidation of acetaldehyde can be improved by the presence in the reaction medium of certain organic liquids, in particular, liquids which dissolve acetic anhy dride but which do not dissolve and are not soluble in water to any considerable extent.

According to the present invention the oxidation of acetaldehyde is effected in the presence of aliphatic hydrocarbons.

As the hydrocarbons there may be employed 1 petroleum fractions such as benzine, petroleum fractions boiling at higher temperatures, e. g. up to 150 or 200 0., and individual aliphatic hydrocarbons, e. g. the pentanes and hexanes. The amount of hydrocarbons should be sufficient to produce a separate, and preferably ous, non-aqueous phase.

The use ofthe aliphatic hydrocarbons in this way ofiers a number of advantages in the manufacture of acetic anhydride by oxidizing acetaldehyde. that the water produced in'the oxidation forms and is maintained in a phase separate from the anhydride produced. and this considerably reduces the loss by hydrolysis of anhydride while the oxidation is proceeding, Furthermore, it is possible to operate the process continuously; thus, the anhydiidc and hydrocarbon mixture may be a continuthus, the-metals may be inthe form of salts of For instance, one result of their use is at intervals, separated from the aqueous phase and fractionally distilled to separate th anhydride and hydrocarbons, the latter then being returned to the reaction zone. Theseparation of thehydrocarbon and aqueous phases may be efiected before orafter the anhydride and hydrocarbon mixture leaves the reaction zone; or part of the aqueous phase may be separated before and part after the mixture leaves the reaction zone. When part or all of the aqueous phase is sepa-'- rated fromthe hydrocarbon phase inside the reaction zone, it may be separately removed from other oxidising gas may be led into a mixture of acetaldehyde and the hydrocarbons under the desired temperature and pressure conditions. In the second, the acetaldehyde in vapour or liquid form may be fed in admixture with, or sepaperature and pressure. Usually, this second method is to be preferred to the first, as it gives a better yield of anhydride and enables the reaction to be more readily controlled.

The reaction conditions preferably employed are similar in the two methods, and are such'as give rise to active and rapid oxidation of, the acetaldehyde. Suspended or preferably dissolved inthe hydrocarbons there may be an active oxidation catalyst, e. g. a copper, cobalt or nickel salt' or mixtures of salts of any two or all three of these metals. Manganese catalysts may be used if desired, but as they are less active, they are preferably employed, if atall, with salts of one or more of the other three metals. The metals may, for example, be present in the form of their acetates, but it is advantageous to employ I salts which are more soluble in the hydrocarbons;

long chain aliphatic acids, e. g. as oleates, stearates or linoleates. 1 A useful mixed catalyst consists of a mixture of cobalt and copper salts of long chain aliphatic acids in which the cobalt:

copper ratio is between about 3:2 and 2:1.

Preferably the oxidation is carried out under superatmospheric pressures bothwith a view to rate into two layers;

sures of 55-65 lbs. per square inch, are preferably employed when the oxidising gas is a diluted oxygen, for example air, and the lower pressures, e. g.

25-35 lbs. per square inch, when undiluted oxygen is used. Similarly, although the reaction can, in

general, be carried out eificientlyover a range of temperatures of about 35-75" C. and even lower temperatures may be employed, it is preferred to 'use the higher temperatures in the range when using a diluted oxygen, for example air.

When working under suitable conditions of oxidising'gas is diluted oxygen, e. g. a gas mixture containing oxygenand for example 5, 10, 20 or 30 times its own volume of an inert diluent gas, for instance .nitrogen, carbon dioxide,

' tion' proceeds the'contents der a gauge pressure" of 60 lbs. to a temperature of about65 C. Into the heated mixture air is forced in the form of fine bubbles. As the reacof the autoclave separate into an aqueous phase containing acetic acid, and a hydrocarbon phase containing acetic anhydride as well as some acetic acid.-

lowed 'to separate into two layers.

hydrocarbon layer acetic 'anhydride is' recovgaseous hydrocarbons such as methane or mixturesthereof. Thediluent gas assists very considerably in the evaporation and removalof the hydrocarbons with entrained. water vapour. Hydrocarbons so removed may, after being separated from the entrained water, he returned to the process. Similarly, when carrying out the oxidation as a batch process, part or all of the water may be removed by using the hydrocarbons as an entraining liquor at the .close of the oxidation. The evaporation may be aided by the passage of oxidising and/or inert gases. 1

Whether the acetaldehyde is already present mixed with the hydrocarbons or is added in vapour or liquid form together or simultaneously with the oxidising gas, the latter is preferably fed into the hydrocarbons in such a manner as toproduce an intimate mixture of gas, acetaldehyde and hydrocarbons. For example, the

oxidising gas may be forced through a rose or a set of fine jets, so as to produce'a cloud of small bubbles in the hydrocarbons. 1f the acetaldehyde is introduced in the form of vapour or liquid, it

also may be caused to form flnebubbles or droplets.

C Another very eflective method consists in carrying out the oxidation in a series of narrow tubes, using-a rapid current of oxidising gas so as to cause the hydrocarbon to froth in the tubes;

the acetaldehyde may be initially present in admixture with the hydrocarbons or may be, introduced together or f simultaneously with the oxidising gas. As indicated above, the oxidation can, .with advantage, be carried out as acontinuousprocess, and the oxidation using narrow tubes is preferably so'performed. Thus, whether or not a part of the hydrocarbon liquoris evaporated during l the oxidation so as to remove part or all of the Y sister, the contents of thetubes'may be run 01! atintervals or continuously and allowed to sepa- The hydrocarbon layer then be distilled tofrecover the .anhydride' produced, and the hydrocarbon returned to the process. The following examples illustrate the invention without limiting it in anyway.

Equal weights of acetaldehyde and benzine containing 0.2% (on the weight of the mixture) of copper oleate are heated in an autoclave, un-

at about hydride from the non-aqueous phase tion'.

After the oxidation these two phases are al- From the ered by distillation, and thebenzine (with any acetic acid it-may contain) is returned to serve in the oxidation ofa further batch of acetaldehyde..

Example 2 ture is about 0.2% of copper oleate. Air is forced into the lower end of each tube and escapes at the upper end through a liquid trap. The tubes are heated to Gil-65 C., and the pressure is kept tube is a liquid overflow where the petroleum fraction and the other liquids present escape. The rate of flow of the liquid through is so adjusted that the liquid"leaving the tubes contains not more than '10% of the initial quantity of acetaldehyde.

' The liquid leaving the tubecomprises a hydrocarbon phase and an aqueous phase. These are allowed to separate. The hydrocarbon phase is separated by fractional carbon (which may contain some acetic acid) andacetic anhydride. The hydrocarbon, with acetic acid contained therein, process.

Eiample 3 Benzine containing copper oleate is caused to flow slowly through a set of narrow tubes .as

and under the same described in Example 2, conditions. At the same time a mixture of parts by volume of air and 25 parts of acetalde hyde vapour is fed into the tubes and passed through the benzine. Unreacted acetaldehyde may be removed from tlon, and acetic anhydride and benzlne are re covered as described in Example 2.

Having described my invention, what I desire to secure by Letters Patent is: I

1. Process for the manufacture of acetic anhvdride, which comprises subjecting acetaldehyde to oxidation by means of free oxygen in the presence of suflicient of a liquid aliphatic hydrocarbon to form a non-aqueous phase containing acetic anhydride, separating said nonphase simulacetic anby distillaaqueousphas'e from. the aqueous taneously produced, and recovering hydride, which subiecting acetaldeof acetic an- I manufacture of acetic anbjecting acetalde- 65 lbs. Towards the upper end of each.

the tubes distillation into hydrois returned to the the spent air by condensafree swa v aaoassc hyde to oxidation by means of free oxygen in the presence of a copper salt as catalyst and aqueous phase simultaneously produced, and recovering acetic anhydride from the non-aqueous phase by distillation.

5. Process for the manufacture of acetic anhydride, which comprises subjecting acetaldehyde to oxidation by means of free owgen in the presence of a nickel salt as catalyst and sufficient o! a liquid aliphatic hydrocarbon to form a i non-aqueous phase containing acetic anhydrlde, separating said non-aqueous phase from the aqueous phase simultaneously produced, and

recovering acetic anhydride from the non-aqueous phase by distillation.

6. Process for themanufacture of acetic anhydride, which comprises subjecting acetaldehyde to oxidation by means of free oxygen in the presence 01 a catalyst comprisinga higher fatty acid salt of a catalytic metal and sufllcient of a liquid aliphatic hydrocarbon to form-a nonaqueous phase containing acetic anhydride, separating said non-aqueous phase from the aqueous phase simultaneously produced, and recovering acetic anhydride from the non-aqueous phase by distillation. 4,;

HENRY DREYFUS. 

